The present invention relates to novel constructions of air filter units of the downflow, HEPA type, and to methods of measuring and adjusting air flow into such filter constructions and of performing leak testing thereof.
In locations requiring air substantially free of all foreign matter, such as for industrial process or medical applications, it is a common practice to provide high efficiency filter modules having media cores capable of removing essentially all particles down to sub-micron size through which all entering air passes. The media is sealed about its periphery to housing structure including a center board dividing the media core into two halves and defining an enclosed chamber or plenum on the upstream side of the media.
Incoming air is directed by a fan through suitable ductwork to the individual filter modules which are suitably supported and sealed to surrounding structure with the downstream face of the filter media forming a portion of the ceiling of the room to which the filtered air is supplied. Air from the ductwork enters the enclosed space on the upstream side of the media through an opening in the top wall of the housing structure. The rate of air flow into the filter plenum is controlled by the position of a damper mounted for adjustable movement with respect to the opening through which air enters from the ductwork.
It is desireable to test such filter modules periodically after installation for leaks in order to verify that the required level of filtration efficiency is being maintained. This has been done in the past by developing standard test fluids, such as D.O.P. smoke, which are placed in suitable concentrations on the upstream side of the filter core while scanning the downstream side with sensing equipment to detect the presence of particles which have leaked through or around the media. Although such functions as dmper adjustment, air pressure measurement, and test smoke injection and upstream concentration monitoring may be performed from the upstream side of the media, they normally may be more conveniently carried out from the downstream side, assuming suitable provision is made in the filter module construction.
In U.S. Pat. No. 3,522, 724, three passageways are formed through the center board of the filter housing, each providing communication between the upstream and downstream sides of the media. The slotted lower end of an elongated rod may be engaged by a screwdriver inserted through the center opening, rotation of the rod serving to adjust the position of the damper, thereby changing the rate of air flow into the filter plenum. For leak testing, test smoke is injected directly into the filter plenum through one of the other openings while the concentration of the smoke on the upstream side of the media is monitored through the third opening. Thus, in order to adjust the position of the damper, inject test smoke into the filter plenum, and monitor the concentration of the smoke within the plenum, three separate and distinct passageways through the media, or, more commonly, the center board dividing the media into two halves, have heretofore been employed.
Each of the three openings is sealed, of course, when not in use for the intended purpose by a plug removable from the downstream side of the filter unit. However, the more openings or passageways which are provided between upstream and downstream sides of the media, the greater the possibility of leaks occurring when the filter is in use and such passageways are presumably sealed. While injecting the test smoke directly into the filter plenum from the downstream side has the advantage of confining the smoke to a single plenum, it is difficult to distribute such smoke evenly over the upstream side; consequently, the concentration of smoke monitored through an opening at a discrete point above the filter media may not accurately reflect the concentration of smoke over the entire upstream face. Furthermore, while adjustment of the damper position from the downstream side of the filter unit to control the rate of air flow from the ductwork into the plenum is convenient, no means have been provided in such filter units for taking measurements from the downstream side indicating the actual rate of air flow at any given time.
A principal object is to provide a HEPA-type air filter having improved leak testing features; more specifically, this object is to provide means for injecting test smoke from the downstream to the upstream side of the media which assures uniform distribution of the smoke over the upstream face.
A still further object is to provide a HEPA-type air filter construction having a single passageway through the media center board through which a wand may be inserted for injection of test smoke from downstream to upstream sides of the filter media while simultaneously monitoring the concentration of such smoke on the upstream side through the same passageway.
Another object is to provide novel and improved methods of effecting damper adjustment, making air pressure and flow measurement, injecting test smoke and monitoring the upstream concentration thereof for leak testing in downflow, HEPA-type air filter modules.
Other objects will in part be obvious and will in part appear hereinafter.